Osteosynthesis plate system

ABSTRACT

An osteosynthesis plate system is particularly well adapted to securely fuse adjacent cervical vertebrae. The plates are for mounting upon the anterior or posterior surfaces of the vertebrae. Plates for mounting on the anterior vertebral surfaces have a concave bone contacting surface and a bone screw locking mechanism integral with each screw hole. Moreover, the bone contacting surface of the plate has a plurality of bone penetrating protrusions to more securely affix the plate to bone. Plates for mounting on the posterior vertebral surfaces also have bone penetrating protections on their bone contacting surfaces. Such plates are formed so as to have a curved bone contacting surface that is concave in the transverse axis of the plate and convex in the longitudinal axis of the plate. The screw holes of such plates are constructed so as to guide a bone screw along a desired angle to improve the anchoring of the screws in bone.

This application is a continuation of application Ser. No. 08/234,240filed on Apr. 28, 1994, now abandoned, which is a divisional ofapplication Ser. No. 07/981,281 filed on Nov. 25, 1992 now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to osteosynthesis plates, and more particularlyto such plates useful to immobilize adjacent vertebral bodies.

Proper healing of injured or damaged skeletal parts requiresimmobilization of the injured skeletal segments to ensure the propergrowth of new osseous tissue between the segments. Insufficientimmobilization can compromise the healing process and may result infurther complications or injury.

Osteosynthesis plates have been used to immobilize adjacent skeletalparts such as bones. Typically, a rigid plate is positioned to spanbones or bone segments that must be immobilized with respect to oneanother. The plate is fastened to the bone, for example with bonescrews, so that the plate remains in contact with the bone and that thebones or bone segments are immobilized.

Such plates have been used to immobilize a variety of bones, and haverecently been adapted for use in fusing and immobilizing adjacentvertebral bodies. The morphology of spinal bone presents uniquechallenges to the design of effective osteosynthesis plates for fusingvertebral bodies. Among the challenges involved in fusing vertebralbodies is the effective installation of a plate that will resistmigration despite the rotational and translational forces it faces. Fora plate to work effectively in such an environment, screws must beproperly positioned and anchored within the bone. Several known platedesigns use elongate, slotted openings for screw placement in the plate.While useful in providing freedom of positioning screws, thiscontributes to the potential for slippage between the plate and screwhead along the longitudinal axis of the plate. Such plates have alsobeen designed to conform to the shape of vertebral bodies which theycontact.

Despite the existence of osteosynthesis plate systems adapted for use infusing vertebral bodies, there remains a need for an osteosynthesisplate system that is able to be securely installed between adjacentbones or bone segments, particularly vertebral bodies.

It is thus an object of the invention to provide an osteosynthesis platesystem which maximizes immobilization of adjacent bones or bonesegments. Another object is to provide such a system adapted for use infusing adjacent vertebral bodies. A further object is to provide anosteosynthesis plate system for placement on the anterior surface ofvertebral bodies to immobilize two or more adjacent vertebrae. It isalso an object to provide an osteosynthesis plate system for placementon the posterior surface of vertebral bodies to immobilize two or moreadjacent vertebrae. Another object is to provide such plate systems ableto be more securely affixed to bone. Other objects will be apparent uponreview of the disclosure that follows.

SUMMARY OF THE INVENTION

The present invention provides an osteosynthesis plate system thateffectively immobilizes adjacent bones or bone segments, and which hasimproved plate rigidity and securement to bone. The plate system of thepresent invention is well adapted for use in fusing adjacent vertebralbodies, particularly vertebrae in the cervical spine.

The plate system of the present invention comprises a rigid, elongateplate member which is adapted to bridge or immobilize adjacent bones orbone segments.

In one aspect of the invention the osteosynthesis plate system isadapted to mount on the anterior surface of vertebral bodies to bridgeand immobilize two or more adjacent vertebral bodies. The plate systemcomprises an elongate plate member having a plurality of substantiallycircular screw holes that extend through the member. Each screw hole hasa substantially spherical seat that is adapted to seat bone screwshaving a spherical head to allow infinite degrees of freedom in theangular orientation of the screw. The bone contacting surface of theplate preferably includes a number of bone penetrating projections thatare adapted to penetrate the bone as a result of compression forcesinduced by fixing the plate to the bone with bone screws. Preferably,the flat regions of the plate lie flush against the bone when the plateis properly installed.

The plate system also includes a locking mechanism that is integral withthe plate member and adjacent one or more of the screw holes. Thelocking mechanism functions to secure the bone screw to the plate memberto inhibit axial and rotational movement of the bone screws seatedwithin the screw holes and anchored in bone. The locking mechanism maycomprise a rotatable cam mounted in the member adjacent one or more ofthe screw holes. The cam preferably has a ovoid shape and is rotatableso as to engage a surface of a screw head seated in a screw holeadjacent to the cam such that axial and rotational movement of the screwis inhibited. Alternatively, the locking mechanism may comprise adeflectable arm that forms a portion of a screw hole seat, and a cammounted in the member adjacent the deflectable arm. The cam is rotatablesuch that upon rotation it imparts a force to the deflectable armcausing the arm to exert a radial force on the screw head to inhibitrotational and axial movement to the screw.

In another embodiment the plate system of the invention is speciallyadapted to be mounted upon the posterior surface of two or morevertebral bodies, particularly in the cervical spine. The plate memberhas one surface which contacts the bone, and another surface which facesaway from the bone. Disposed within the plate are at least two, spacedapart screw holes having substantially spherical countersinks, each ofwhich is adapted to receive a bone screw. Preferably, the screw holesare aligned with each other about the longitudinal axis of the platemember. In a preferred embodiment the screw holes are oriented atdesired angles with respect to the longitudinal and transverse axes ofthe plate member to ensure proper positioning of the bone screw withinthe hole, and thus the proper angular trajectory of the screw into thebone. The bone contacting surface of the plate member includes aplurality of projections, each of which is adapted to penetrate thecortical layer of bone to improve the bone/plate interface. Compressionforces which result from the use of screws in fixing the plate to thebone will cause the projections to penetrate the bone such that flatareas of the bone contacting surface are substantially flush with thebone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an anterior osteosynthesis plate of the presentinvention affixed to the anterior surface of the cervical spine.

FIG. 2 is a view of the bone contacting surface of the anterior plate ofFIG. 1.

FIG. 3 is a view of the non-bone contacting surface of the anteriorplate of FIG. 1.

FIG. 4 is a longitudinal sectional view along lines B--B of the plateshown in FIG. 3.

FIG. 4A is a detail view of a portion of the plate shown in FIG. 4.

FIG. 5 is a transverse sectional view, along lines D--D of the plateshown in FIG. 3.

FIG. 6A is a view of a screw locking mechanism useful with the anteriorplate of the present invention, in an unlocked position.

FIG. 6B is a view of a screw locking mechanism useful with the anteriorplate of the present invention, in a locked position.

FIG. 7A is a view of another embodiment of a screw locking mechanismuseful with the plate of the present invention, in an unlocked position.

FIG. 7B is a view of another embodiment of a screw locking mechanismuseful with the plate of the present invention, in a locked position.

FIG. 8 illustrates a pair of posterior osteosynthesis plates of thepresent invention affixed to the posterior surface of the cervicalspine.

FIG. 9 illustrates the bone contacting surface of a posteriorosteosynthesis plate of the present invention.

FIG. 10A is a side view of the plate of FIG. 9.

FIG. 10B is a view of the non-bone contacting surface of the plate ofFIG. 10A.

FIG. 11 is a detailed view of area B of FIG. 10A.

FIG. 12 is a sectional view along lines 12--12 of the plate illustratedin FIG. 10A.

FIG. 13A is a side view of a posterior osteosynthesis plate of thepresent invention adapted for use at cervical vertebrae 2 and below.

FIG. 13B is a view of the non-bone contacting surface of the plate ofFIG. 13A.

FIG. 14 is a sectional view along lines 14--14 of the plate of FIG. 13A.

FIG. 15 is a view of the non-bone contacting surface of an alternativeembodiment of a posterior plate.

FIG. 15B is a side view of the plate illustrated in FIG. 15A.

DESCRIPTION OF THE INVENTION

The osteosynthesis plates of the invention are useful for fixing andimmobilizing adjacent bones and/or bone segments. The plates are bestadapted for use in fusing adjacent vertebrae, particularly the cervicalvertebrae. The plates are adapted to mount either on the anterior orposterior surface of the vertebrae, preferably secured to the vertebraeby bone screws. The plates are intended to immobilize the adjacentskeletal segments to promote healing. As a result of such immobilizationnew osseous tissue will grow between the adjacent segments, resulting infusing of the segments.

FIG. 1 illustrates an osteosynthesis plate system 10 adapted to beaffixed to the anterior surface of the cervical spine. The system 10comprises an anterior osteosynthesis plate 12 having positioned thereina number of bone screws 14 that extend through screw holes 19, 20 in theplate to allow the screws 14 to compress the plate against the bone whenanchored in bone. The bone screws 14 may be secured in the plate 12 by alocking mechanism 15 that inhibits rotational or axial movement of thescrew. As illustrated, a locking mechanism 15 preferably is adjacenteach screw hole 19, 20 to secure the screw 14 to plate 12.

FIGS. 2 through 5 further illustrate the osteosynthesis plate system 10of the invention. As illustrated in FIG. 2, the plate 12 has a texturedbone-contacting surface 18 that features a plurality of bone penetratingprojections 26. The bone penetrating projections 26 preferably compriseparallel, spaced rows of short protrusions having a triangular ortooth-like shape. The projections 26 are disposed in substantiallyevenly spaced rows 27 along the long dimension of the plate, with eachrow preferably extending parallel to the transverse axis 23 of theplate.

As illustrated in FIGS. 4 and 4A the projections 26 are substantiallytriangular or tooth-shaped objects having acute angles of approximately60° and 30°. A base 28 is formed by the surface 18 of the plate and ahypotenuse 30 slopes upwardly toward the thickness (or Z) axis midline32 of plate 18. Preferably the short leg 34 is approximately 0.5 mm inheight. The projections preferably are oriented with the cutting edges29 facing toward the thickness axis midline 32 of the plate.

The non-bone contacting surface 36 of plate 12, as shown in FIG. 3,includes a plurality of screw holes 19, 20 that extend through the plateand which are adapted to seat bone screws. Preferably a lockingmechanism 15 is disposed adjacent each screw hole 19, 20. The lockingmechanism 15 may comprise a rotatable cam member 24 permanently housedin an aperture 22. The screw holes 19, 20, as illustrated, preferablyare substantially circular in shape so as to provide fixed positionholes for the placement of screws. The screw holes further havespherical seats, which correspond to a spherical head on the screw, topermit better seating of the screw within the screw hole and infiniteangular degrees of freedom in orientation of the screw. The circularscrew holes 19, 20 are advantageous as the plates are less prone tolongitudinal movement relative to the screws, as sometimes occurs withslot-like or elongate screw holes.

As noted above, the plate system 10 of the invention comprises a lockingmechanism 15 that inhibits axial and rotational movement of bone screws14 once the plate is affixed to vertebral bodies. FIGS. 6A and 6Billustrate one embodiment of the locking mechanism 15 in which arotatable cam member 24 is permanently housed in an aperture 22 adjacentscrew hole 20. With the cam in the unlocked position, as illustrated inFIG. 6A, a screw (not shown) can be inserted into screw hole 20 foranchoring within the bone. Once the screw is fully inserted, cam 24 isthen rotated approximately 90°, as shown in FIG. 6B, such that itssurface impinges upon a screw inserted in screw hole 20, thus inhibitingrotational and/or axial movement of the screw. Such a locking systemintegral with the osteosynthesis plate is advantageous in that it doesnot require a surgeon to manipulate small mechanical parts that must beinserted on the plate for locking. Rather, a permanently installed camis simply rotated to a locked position to engage the screw.

FIGS. 7A and 7B illustrate a locking mechanism 35 which forms anotherembodiment of the invention. The locking mechanism comprises screw hole20 adapted to receive a bone screw (not shown) and a rotatable cam 42permanently housed in aperture 40. A portion 37 of the wall of screwhole 20 forms a deflectable arm 44. Adjacent arm 44 is an arcuate slot38 that connects with aperture 40.

As shown in FIG. 7A, the locking mechanism 35 is in the unlockedposition. In this position a screw may be inserted into the screw hole20 and affixed to bone. Once fully affixed, cam 42 is rotatedapproximately 90° to a locked position, as shown in FIG. 7B, therebyexerting a force on deflectable arm 44. The force exerted by cam 42 onarm 44 causes the arm 44 to impinge upon a screw head 46 mounted inscrew hole 20 to inhibit axial and rotational movement of the screw.

As noted, cam members 24, 42 preferably are permanently installed on theplate. This can be accomplished in a variety of ways readily understoodby one having ordinary skill in the art. For example, the cam can have astem with a tubular rivet at one end enabling the cam to be staked ontothe plate.

The plate 12 preferably is constructed so as to conform to the shape ofthe anterior surfaces of the vertebrae that it will be mounted upon.Ideally, upon compression of the plate to the bone through the action ofbone screws, the bone penetrating projections 26 will become embeddedwithin the cortical layer of bone and the bone contacting surface 18will be substantially flush against a major portion of the bonesegments. The penetration of projections 26 within bone is beneficial inthat it affords a more secure fit of the plate to the bone. Moreover, itis believed that the contact of projections 26 with the periosteum mayprovide sufficient antagonism to evoke a healing response, which mayenhance the formation of the solid fusion mass.

As illustrated in FIGS. 4 and 5, the plate 12 is curved along both itslongitudinal and transverse axes such that the bone contacting surface18 is concave. This configuration of the plate 12 is advantageous toenable the plate to conform to the shape of the vertebrae. In apreferred embodiment, the length of the plate 12 along the longitudinalaxis preferably results from extending an arc from a minimum of about 7°to a maximum of 25° along an 8.0 inch radius. The width of the platealong the transverse axis preferably results form extending an arc ofapproximately 52° along a 0.75 inch radius.

Plate 12 is intended to be positioned with its longitudinal axis 31colinear with the spinal midline, and to be mounted on the anteriorsurface of the vertebral bodies. Ideally, the longitudinal length of theplate will be sufficient to completely cover the anterior surface of thevertebral bodies from the top front edge of the most cephalad body tothe lower front edge of the most caudal body. The length of the plate aswell as the number of screw holes in the plate will, of course, varydepending upon the number of vertebral bodies to be fused. The plateillustrated in FIGS. 1 through 3 is intended to span three vertebralbodies and includes three pairs of adjacent, longitudinally spaced screwholes in which each member of the pair is disposed on one side of thelongitudinal midline of the plate 12. Further, the plate preferablyincludes two screw holes 19A, 19B disposed on the longitudinal midlineof the plate and on opposite sides of the transverse midline of theplate. Holes 19A, 19B are intended for use in securing optional bonegraft pieces placed between adjacent vertebral bodies.

While the plate 12, illustrated and described above, is adapted to spanthree vertebral bodies, plates may be designed to span anywhere from twoto four vertebral bodies. The dimensions of plates intended to span agreater number of vertebral bodies as well as the number of screw holesto be placed in such plates will be readily understood by one havingordinary skill in the art. For example, a plate intended to secure twovertebral bodies can have a length of about 24 to 36 mm with four bonescrew holes and one graft screw hole. Similarly, a plate intended tosecure three vertebral bodies can have a length of about 38 to 54 mmwith 6 bone screw hoes and two graft holes.

FIG. 8 illustrates a pair of osteosynthesis plates 100, affixed to theposterior surfaces of and fusing the C₃, C₄ and C₅ cervical vertebrae.As illustrated, the osteosynthesis plates 100 intended for mounting onthe posterior surface of vertebral bodies are designed to be used inpairs, mounted on the lateral masses of the vertebrae on either side ofthe spinous process 101. The longitudinal axes of plates 100 aresubstantially parallel to the spinal midline. Ideally the plates'longitudinal length will be sufficient to completely cover the surfaceof the lateral mass and facets, from the superior edge of the mostcephalad body to the inferior edge of the most caudal body. Each plate100 has a plurality of screw holes 102, each adapted to receive a bonescrew 104, which is adapted to be affixed within the lateral mass of onevertebral body to be joined. The screw holes 102 preferably are alignedalong the longitudinal axis of the plate 100.

The bone-contacting surface 106 of the posterior plate 100, asillustrated in FIGS. 9-11, features a textured surface. The surface 106has a plurality of parallel, spaced rows of short protrusions 108 whichare triangular or tooth-like in shape. The tooth-like rows preferablybegin on opposite ends of the long dimension of the plate 100 and recurin spaced intervals toward the center of the plate. The protrusions 108are oriented with their cutting edges 109 facing toward the thickness(or Z) axis midline 111 of the plate 100. As illustrated in FIGS. 10Aand 11, each protrusion 108 is shaped substantially as a right trianglehaving a base 110 corresponding to the bone-contacting surface 106 and ahypotenuse which extends upwardly toward the thickness axis midline 111of the plate 100. The short leg 114 has a height of approximately 0.5 mmand faces toward the thickness axis midline 111 of the plate 100.

Protrusions 108 are designed to penetrate the cortical layer of thevertebral bodies upon a compressive force induced by the bone screws 104when anchored to bone. When a plate 100 is installed, the embedded teethprovide a foothold to better enable the plate to resist migration duringtranslation and rotation. Moreover, the bone-contacting surface 106should lie substantially flush with the vertebral body when the plates100 are installed and protrusions 108 are embedded in bone.

Screw holes 102, as illustrated in FIGS. 9 and 10, are substantiallycircular in shape and preferably are slots with hemicircular ends. Screwholes 102 have spherical countersinks to prevent translation of thebodies through plate slippage. The holes 102, as noted above, arealigned in the longitudinal axis of the plate. Screw holes 102 may be ofother shapes as well, including ovoid and elliptical.

Referring to FIGS. 8 and 10A through 13B, the plate 100 has a lateralside 118 and a medial side 120. Each screw hole 102 preferably has asubstantially wedge-shaped transverse cross section. The wedge shape ofthe screw holes 102 can be achieved by each hole 102 being formed in awedge-like protrusion 122 integrally formed on the dorsal (non-bonecontacting) surface 124 of plate 100. For screw holes adapted to seat abone screw to be anchored at the C₃ to C₇ vertebrae, the wedge-likeprotrusion 122 has a raised portion 126 on the lateral side 118 of theplate which tapers to the medial side 120 of the plate. The screw holes102 are formed angularly in the wedge-like protrusion 122 such thatscrews may be seated in the vertebral bodies at an optimal angle.

The plates 100 illustrated in FIGS. 10A, 10B, and 12 are intended formounting upon cervical vertebrae 3 through 7. The screw holes 102 formedin such plates preferably should have a cephalad angle (α_(c)) oforientation of approximately 15°. Similarly, the wedge shaped protrusion122 enables screw holes 102 to be formed at a lateral angle (α_(C)) ofabout 20°. Accordingly, bone screws intended to be anchored in the C₃ toC₇ vertebrae will preferably enter the vertebral body projecting in thecephalad direction at an angle of about 15° relative to the thicknessaxis 142 of each hole. The screw also projects in the lateral directionat an angle of 20° relative to the thickness axis 142 of each hole.

FIGS. 13A, 13B and 14 illustrate a plate having one screw hole 116specially adapted to seat a screw intended to be anchored in the C₂vertebra. Screw holes 128, 130, and 132 are constructed as describedabove and are intended to seat screws to be anchored in the C₃, C₄ andC₅ vertebrae, respectively.

Due to the morphology of the C₂ vertebra, a bone screw should bedirected into this vertebral body at a medial and cephalad angle, ratherthan at a lateral and cephalad angle. Accordingly, the wedge-likeprotrusion 134 within which hole 116 can be formed having a raisedmedial side 136 and a raised cephalad side 138. The geometry of hole 116enables a bone screw to project into the lateral mass of the C₂ vertebrain the cephalad direction at an angle (α_(c)) of about 25° relative tothe thickness axis 142 of each hole. The plate also projects in themedial direction at an angle (α_(m)) of about 10° relative to thethickness axis 142 of each hole.

FIGS. 10A, 12, 13A and 14 illustrate the desired curved geometry of thebone contacting surface 106 of the plate 100 that maximizes the abilityof the plate to conform to the shape of the posterior surfaces of thecervical vertebrae. Preferably, the plate 100 is curved about itslongitudinal axis such that the bone contacting surface is convex, andabout its transverse axis such that bone contacting surface 106 isconcave. In a preferred embodiment the longitudinal curve of plate 100is along an arc of about a 165 mm radius. The plate preferably has atransverse curve along an arc of a 22 mm radius. One skilled in the artwill readily appreciate that the curve of the plate in both thelongitudinal and transverse axis may be modified to improve itsconformity to the shape of the vertebrae bodies.

FIGS. 15A and 15B illustrate an alternative embodiment of plate 100,having a thicker profile. The plate 100 illustrated in FIGS. 15A and Bhas a thickened, or raised lateral side 140, which replaces theindividual wedge-like protrusions surrounding each screw hole 102 as inFIGS. 8 through 14.

Plate 100 illustrated herein has four screw holes and is adapted to fusefour vertebrae. Plates can be designed with a greater or lesser numberof screw holes to fuse more or less vertebral bodies. Typically, platescan have between about two and five screw holes each for fusing from twoand up to five vertebrae.

One of ordinary skill in the art will appreciate that the plates 12, 100of the invention can be made of a variety to high strength, biologicallycompatible materials that preferably are compatible with MRI techniques.Useful materials include polymers, stainless steel, titanium andtitanium alloys. A currently preferred material is a titanium-aluminumalloy having 90% titanium and 6% aluminum, and 4% vanadium.

Various modifications may be made to the plate system of the inventionwithout exceeding the intended scope of the claims. For example, thebone penetrating projections may assume a different geometry and may beoriented so as to be parallel with the longitudinal axis of the plates.

What is claimed is:
 1. An osteosynthesis plate system, comprisinga rigid, elongate plate member adapted to mount upon the posterior surfaces of two or more vertebral bodies to bridge and immobilize adjacent vertebral bodies, the member having a first, bone contacting surface and a second, non-bone contacting surface and the member being curved such that the first surface is concave in the transverse axis of the member, with the apex of the concavity being in the direction of the second surface, and convex in the longitudinal axis of the member such that the first surface is capable of contacting two or more vertebral bodies upon which the plate member is mounted; at least two screw holes having substantially spherical countersinks and each being adapted to receive a bone screw, the holes being in substantial alignment with one another about the longitudinal axis of the member and being oriented at an angle in the cephalad direction in the range of 15° to 25° and at an angle in the medial or lateral directions in the range of 10° to 20° relative to a thickness axis of the member running from the first surface to the second surface to ensure proper positioning of the bone screw within the hole at a desired cephalad angle and at a desired medial or lateral angle relative to the thickness axis of the member, angular orientation of the screw holes in the medial direction being achieved by a wedge-shaped protrusion integrally formed on the medial side of the plate adjacent each of the screw holes and angular orientation of the screw holes in the lateral direction being achieved by a wedge-shaped protrusion formed on the lateral side of the plate adjacent each of the screw holes; and a plurality of bone penetrating projections disposed on the first surface of the member.
 2. The plate system of claim 1 wherein the member is adapted to mount upon the posterior surface of the vertebral bodies, adjacent to the spinous process thereof, and the member has between 2 and 5 screw holes, each being adapted to guide a bone screw into the lateral mass of a vertebral body, with one bone screw positioned within each vertebra being fused.
 3. The plate system of claim 1 wherein the bone penetrating projections are formed of discontinuous structures, each extending traverse to the longitudinal axis of the member.
 4. The plate system of claim 3 wherein each structure has a substantially triangular cross section.
 5. The plate system of claim 4 wherein a cutting surface of each structure faces toward the thickness axis midline of the member.
 6. An osteosynthesis plate system, comprisinga rigid, elongate plate member adapted to mount upon the posterior surfaces of two or more vertebral bodies to bridge and immobilize adjacent vertebral bodies, the member having a first, bone contacting surface and a second, non-bone contacting surface and the member being curved such that the first surface is concave in the transverse axis of the member, with the apex of the concavity being in the direction of the second surface, and convex in the longitudinal axis of the member such that the first surface is capable of contacting two or more vertebral bodies upon which the plate member is mounted; at least two screw holes having substantially spherical countersinks and each being adapted to receive a bone screw, the holes being in substantial alignment with one another about the longitudinal axis of the member and being oriented at an angle in the cephalad direction in the range of 15° to 25° and at an angle in the medial or lateral directions in the range of 10° to 20° relative to a thickness axis of the member running from the first surface to the second surface to ensure proper positioning of the bone screw within the hole at a desired cephalad angle and at a desired medial or lateral angle relative to the thickness axis of the member; a plurality of bone penetrating projections disposed on the first surface of the member; wherein the member is adapted to mount upon the posterior surface of the vertebral bodies, adjacent to the spinous process thereof and the member has between 2 and 5 screw holes, each being adapted to guide a bone screw into the lateral mass of a vertebral body, with one bone screw positioned within each vertebra being fused; and wherein the member is adapted to join at least two adjacent cervical vertebrae from the third to the seventh cervical vertebrae, and the screw holes are angularly oriented within the member such that a bone screw inserted within the screw hole will project in the cephalad direction at an angle of about 15° relative to the thickness axis of the hole and in the lateral direction at an angle of about 20° relative to the thickness axis of the hole. 